There is known a threaded member tightening tool for tightening threaded members, e.g. screws and nuts, which is configured to adjust the threaded member tightening torque to an appropriate magnitude.
A tool disclosed in Patent Literature 1, for example, has a mechanical clutch mechanism to perform a torque adjustment with the clutch mechanism. Specifically, when a torque greater than a predetermined value is applied to the clutch mechanism after a threaded member has been seated, the clutch mechanism is activated to cancel the mechanical connection between a motor and a threaded member engaging unit, e.g., a screwdriver bit, so that a torque greater than the predetermined value will not be applied to the threaded member.
A threaded member tightening tool using the above-described clutch mechanism is likely to become large in size and heavy in weight due to having a mechanical structure constituting the clutch mechanism. Accordingly, there has also been developed a threaded member tightening tool that is not provided with a mechanical clutch mechanism but, instead, configured to adjust the torque when a threaded member has been seated by electrically detecting the torque applied to the motor by using an electric current sensor detecting an electric current flowing through the motor, or using a torque sensor, in order to make the tool compact in size and light in weight. In such a tool, the drive of the motor is stopped upon detecting the completion of tightening of a threaded member by electrically detecting the torque. In this regard, however, inertia force acting on the motor, a speed reducer, etc. is borne by the threaded member. Therefore, an excessive force may be applied to the threaded member, which may cause breakage of the threaded member or an object to be fastened with the threaded member. Accordingly, another conventional threaded member tightening tool is configured as shown in Patent Literature 2, for example. That is, screw tightening is started at a relatively high rotational drive speed at first, and before the threaded member is seated, the rotational speed is reduced to a speed at which no excessive torque will be applied to the threaded member. Thereafter, the threaded member is tightened completely.